Thulium-Doped Fiber Amplifier Development for Power Scaling the 2 Micron Coherent Laser Absorption Instrument for ASCENDS

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1 Thulium-Doped Fiber Amplifier Development for Power Scaling the 2 Micron Coherent Laser Absorption Instrument for ASCENDS Mark W. Phillips Lockheed Martin Coherent Technologies 135 South Taylor Avenue, Louisville CO mark.w.phillips@lmco.com /3/2011 1

2 Acknowledgments Work funded under NASA/ESTO Grant ACT ( CLASS ACT ) Co-Investigators Gary Spiers & Bob Menzies (JPL) LM Co-Workers Tahllee Baynard, Mike Hinckley, Robert Nichols, John Hobbs, Ross Mackes, Nathan Woody 2

3 CLASS ACT Primary Objective Demonstrate power-scaling of 2 micron (FM)CW solution for ASCENDS From 100mW (airborne CO2 LAS) to >5W (ASCENDS with 75cm aperture), using fiber amplifier technology Maintain absolute frequency control (total drift) <1MHz Power scaling identified as #1 risk for 2 micron approach ASCENDS Workshop (Bar Harbor, 2008) 50x power scaling supports aperture reduction on aircraft instrument Major reduction in instrument integration time for improved spatial resolution (Coherent CW approach is a speckle-integrated measurement) 3

4 CLASS ACT Secondary Objectives Perform radiation tests and sensor design study Determine suitability of fiber amplifiers and other fiber components for ASCENDS Radiation Testing ( 60 Co) of Key Fiber Amplifier Components (On Transmit) EOM3 On Mon 2 EOM4 (Off Transmit) (Off LO) AOM3 Off LO Mon Off RCVR 95:5 95:5 90:10 ISO1 ISO2 Off Mon 2 Circ 1 FBG1 Fiber Amp ISO3 Circ 2 ISO4 FBG2 T/R Optics (Free Space) PBS BEX?/4 To Telescope Fiber-Based Sensor Design Study Point of Departure Design from CO2 LAS to ASCENDS (On LO) AOM2 CO2 LAS Transceiver Developed by LM for JPL airborne instrument On LO Mon On RCVR 90:10 4

5 2-Step Power Scaling Approach LM METEOR Laser Tm,Ho:YLF (2051nm) Power: mW (SLM) 20GHz tuning range Linewidth: 10kHz/ms (free-running) 1 ASCENDS Sensor Power: 5W (SLM) per transmit channel Linewidth: < 1MHz long-term absolute frequency lock Aperture: 75cm dia CO2 LAS Transceiver Power: 100mW (SLM) for On-line and Off-line transmit channels Linewidth: < 1MHz long-term absolute frequency lock Aperture: 10cm dia 2 Single Frequency Fiber Amplifier Tm:glass (2 micron operation) Power: >5 W out for >30mW signal input (saturated output) Redundant pump diodes Input and output optical isolation Test Device procured from Nufern 5

6 Tm:Glass Fiber Amplifier (Nufern) (enclosure removed) Single frequency METEOR Laser Image Courtesy of Nufern Image Courtesy of Nufern >8W output power demonstrated when seeded with 50mW single frequency METEOR Laser 6

7 2 Micron Amplifier Performance Results (using stand-alone METEOR Laser) No observed rollover in power due to nonlinear effects at this power level Input single frequency signal power: nm 7

8 Fiber Amplifier Optical Spectrum Optical 8.5 W output power Signal to Noise >38dB No ASE or lasing observed over amplifier gain bandwidth (1960 nm to 2060 nm) 8

9 Fiber Amplifier Optical Design Image Courtesy of Nufern 1 st Stage Amp 2 nd Stage Amp 1 pair of pump diodes active at any time in each amplifier stage Triple redundancy in pump diodes 9

10 Fiber Amplifier Hardware in Test (with CO2 LAS Optical Front End) CO2 LAS Sensor (Single frequency laser input source with 1MHz absolute lock control) Amplifier Control PC and GUI Tm Fiber Amplifier Amplifier output attenuated and coupled back into CO2 LAS frequency lock assembly 10

11 Optical Configuration using CO2 LAS Optical Front End as Input to Fiber Amplifier Power Monitor 98:2 /2 /2 HR Laser 2 (On-Line LAS) (FOL to Laser 1) FOL Beat Detector 2 Heterodyne Detector 1 90 AOM Laser 1 (Locked to CO2 Line-Center) Heterodyne Detector 2 FOL Beat Detector 1 Laser 3 (Off-Line LAS) (FOL to Laser 1) /2 P P EOM 10 (not diffracted) 95:5 90:10 90:10 90:10 90:10 50:50 EOM 95:5 95:5 90:10 90:10 /2 Laser 2 Power Monitor Iris Laser 1 Power Monitor Laser 3 Power Monitor 2x Beam Expander CO2 Gas Cell /4 2x Beam /2 Expander /4 To On-line Telescope FM Beat Detector To Off-line Telescope Tm Fiber Amplifier CO2 LAS Sensor (Single frequency laser input source with 1MHz absolute lock control) 11

12 Fiber Amplifier Performance (Power, Frequency Spectrum and Wavelength) 12

13 Reference MO Lock Performance Lock Discriminant Slope Calibration Isotope peaks are 2.88 GHz apart Slope is 5.34 mv/mhz Locked Error Signal Amplitude: 20mV/div scale Total amplitude p-p is <4 mv (< 700 khz p-p absolute stability) Mostly 60Hz noise that can be further filtered out RMS uncertainty < 270 khz 13

14 Frequency Uncertainty between Reference Laser and 5W Amplifier Output 3dB linewidth = 400kHz 70dB 3dB linewidth = 400kHz 70dB Heterodyne linewidths (@ -3dB) =400kHz De-convolving linewidths of reference laser and amplifier output: Amplifier output linewidth ~ 275 khz Limited by CO2 LAS lock performance CO 2 LAS (100mW) (Instrument data for comparison) Amplified CO 2 LAS (5W) 14

15 Fiber Amplifier Absolute Lock Performance At >5W output power, total absolute frequency uncertainty of fiber amplifier output < 700kHz p-p absolute reference lock khz beat between reference laser and amplifier output (deconvolved) ~ 1MHz peak-to-peak Primary Objective Met 15

16 Radiation Testing of Fiber Amplifier Key Components 1W fiber amplifier tested in breadboard format with extended patch leads between major sub-assemblies Enables insertion of separate sub-assemblies into radiation chamber Represents first amp stage of full 5W amplifier design Plate 1 Plate 3 Plate 2 16

17 Radiation Test Configuration Active fiber element (Plate 3) placed in primary radiation chamber ( 60 Co) Plates 1 and 2 placed in shielded secondary (lower) chamber 17

18 Radiation Test Diagnostics Optical Assembly Single Frequency Laser Source Self-heterodyne detection: In-band Signal Monitor Fiber Component under Test Total Output Power and Polarization Monitor Power Attenuator 18

19 Radiation Test Configuration (2) Diagnostics Optical Assembly (cart-mounted breadboard) Radiation chamber in background 60 Co (gamma source) 19

20 Radiation Test Results Preliminary (Amplifier unpumped during dose accumulation) Measure self-heterodyne signal (in-band power) and total output power as function of cumulative radiation dose Initial test shows 3dB in-band signal degradation at 10 krad dose and accelerated degradation at dosage beyond 10 krad Initial test set-up susceptible to optical misalignment between data points Re-running test with improved set-up to corroborate initial results and to test with amplifier operational during radiation dosage accumulation 20

21 Summary >5W output power with <1MHz absolute frequency lock knowledge demonstrated in lab #1 risk identified for 2 micron CW approach to ASCENDS Power scaling also enables finer spatial resolution airborne measurements (more rapid speckle integration) Radiation testing of fiber amplifier key components using 60 Co gamma source Preliminary results (non-operational during dose accumulation) indicate current fiber susceptible at cumulative doses lower than 10 krad Setting up to perform equivalent test with operational fiber amplifier Future test plans Evaluate fiber changes for improved radiation tolerance Identify and radiation-test other key fiber components for space-based sensor compatibility 21

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